Psychophysical tests in optometric practice: assessing defective dark adaptation Flashcards
what 3 things are psychophysical procedures developed for
- measure thresholds
- aim to give accurate results
- aim to give reliable results
give an example of when you measure threshold in practice
resolution threshold or contrast threshold i.e. being able to detect some kind of stimulus
what 3 key points have to be considered when carrying out psychophysical procedures
- with the fewest number of stimulus presentations (ask least amount of questions)
- without requiring sophisticated judgements from the observer (just yes or no, left or right etc = basic)
- with minimal opportunities for psychological bias (don’t want their psychology to affect results)
what does a visual psychophysical procedure involve
presenting visual stimuli that vary along one of a number of possible dimensions
what is the ideal procedure when carrying out a psychophysical test
all but one parameter kept constant
i.e. just change one parameter of vision we’re interested in, e.g. if measuring VA’s, the only parameter we want to change is the size of the letter & so contrast, spacing of letters etc must stay constant. so the only thing we are trying to adjust is the one thing we’re trying to take a measure of
give 3 example of visual psychophysical procedures and what they measure
- visual acuity measures threshold for resolution (MAR)
- amplitude of accommodation measures threshold for near resolution
- near point of convergence measures threshold for BSV
how is the Bailey Lovie (logmar) chart a better psychophysical test of resolution than the Snellen chart
because the only factor that changes as you go down is the size of the letters
why is the Snellen chart not as good as a psychophysical test for resolution than the Bailey Lovie LogMar chart
because the crowding of letters also changes as you go down the chart
list the 3 classical psychophysical methods
- method of adjustment
- method of limits
- method of constant stimuli
what is the threshold when using the method of adjustment technique
point where stimulus just seen (or average of ascending and descending trials)
explain how an ascending trial works
start with a target that can’t be seen & increase its intensity until its just visible = threshold
explain how a descending trial works
start with a target that can definitely be detected & decrease its intensity until it just disappears = threshold
describe how a method of adjustment psychophysical technique is used in optometric practice
measure range for near vision using RAF rule, push up & push down method
“bring text towards you until the text becomes blurry”
= reading the chart until the text becomes blurry and then move it away until it becomes blurry again.
here the px is changing the stimulus i.e. method of adjustment
list the 3 advantages of method of adjustment
- fastest technique of measuring threshold
- good for obtaining an estimate
- subject likely to pay more attention - as your asking them to make the adjustment
what is the disadvantage of method of adjustment
psychophysical bias - difference in confidence
e.g. confidence px will try hard & a nervous px will not try as hard to meet threshold limit
describe how determining the threshold with method of limits psychophysical technique works
adjustment of intensity in regular steps until threshold is reached. can be ascending or descending (not gradual, but step wise, from seen to unseen and vice versa)
when is an ascending limits method used in optometric practice, how and why
often used in dark adaptometry = the measurement of thresholds during dark adaptation
i.e. measuring a px’s threshold as they adjust to the dark, so dont want to present a light stimuli as that will light adapt the px, so start with ascending dim light & increase until they can just see it. start with something below their threshold & increase intensity until its just detected.
when is a descending limits method used in optometric practice, how and why
in letter charts e.g. pelli robson (contrast chart) or Bailey Lovie LogMar chart
start with a stimulus intensity e.g. contrast that the px can definitely detect & decrease the contrast step wise down the chart until can’t detect the letters anymore, threshold = last group of letters which able to read. also used in va charts where decrease the size of letters step wise as go down.
because good for building patient confidence
what is an advantage of method of limits psychophysical technique
quick and simple
list the 3 disadvantages of method of limits psychophysical technique
- errors of habituation - if constantly have to give the same answer, they get used to it e.g. in cross cyl px will keep saying 1 as habitual to it
- subject know what to expect - errors of anticipation, px guesses what next one will look like so respond based on what their expecting rather than what they see
- differences in confidence between subjects (psychophysical bias) - when measuring va’s, some px will guess the letters & get a better result & others will respond they’re really sure with what they can see
name a modified method of limits
staircase method
explain how the staircase method works using intensity as an example
keep changing direction of intensity of our stimulus e.g. if start with a stimulus that px can definitely see and decrease the intensity, but they can still see it, decrease the intensity again & then they can’t see it anymore, but instead of taking that as our threshold, we make the stimulus brighter again, so start increasing brightness in steps until the stimulus reappears again & then change direction & make it dimmer, so keep zig zagging around the threshold by increasing and decreasing the intensity. each of the points where we changed the direction of the threshold = the average of reversal points i.e. R1+R2+R3+R4+R5+R6+R7/7
how do you achieve more accurate results from the staircase method if limits
the more reversals of threshold give more accurate results
how does the more reversals of threshold in the staircase method of limits give more accurate results
minimises affect of habituation and anticipation
what is a disadvantage to producing more reversals of threshold in the staircase method of method of limits
increases fatigue
which type of optometric practice is the staircase method of method of limits used
visual field screeners
how and why does the visual field screener use the staircase method in method of limits
to make the results more accurate it has lots of staircases that are out of sync with each other.
e.g. in a full threshold test, each individual stimulus will have its own staircase going on in that point & the screener goes from one spot & to another spot & each are at different points of the staircase, so px can’t guess whats coming next. however this takes a long time so have settings such as sita and zata fast
explain how the method of constant stimuli psychophysical technique works
you have a load of different stimulus intensities/brightness & present these repeated amount of times, each of these are different intensities but all jumbled up, then measure the % of times the px detects the stimulus at each intensity.
so if you present each of these 10x and work out how many times they detected each of these intensities. e.g. if its a very dim stimulus = they detect 0% of the time & if very bright stimulus = detect 100% of the time i.e. overtime you show them that spot they will detect it, but at threshold will detect it 50% of the time, so will see it half of the time and not the other half.
how would you plot a graph of the method of constant stimuli results
y axis = frequency of seeing of number of times that each of these spots is seen, against x = the intensity of the stimulus
what is the advantage of method of constant psychophysical technique
most accurate
list the 3 reasons why the method of constant stimuli psychophysical technique is the most accurate technique
- patient can’t anticipate next stimulus
- lots of presentations of each stimulus intensity
- can include blank frames and frames well above threshold to check reliability (false +ve and -ve)
what are the 2 disadvantages of method of constant stimuli psychophysical technique
- still problems with difference in confidence (psychophysical bias)
- time consuming
what is the consequence of the method of constant stimuli psychophysical technique being time consuming
it is not used in optometric practice
in what is the method of constant stimuli psychophysical technique mainly used in
research
what are the 4 things that cause inaccuracies of the observer
- psychological bias (decision criterion)
- fatigue
- attention and motivation
- habituation to the task
all cause changes in threshold continuously with real people
what is every psychophysical test performed in practice a balance between
getting the most accurate threshold e.g. for VA or visual fields, without fatiguing to patient too much
what do you look at when measuring dark adaptation
how fast the threshold drops in the dark
so if you expose a px to a very bright light & then put them in the dark = their threshold is initially very high & when they stay 1/2 an hour in the dark, the threshold gradually drops down & down
describe the shape of the graph when measuring dark adaptation and explain why
a bisphasic shape
because initially cones determine the threshold as they recover more rapidly than rods in the dark & then after 10 minutes theres a kink in the curve which is the rod cone break, after that the rods determine the threshold for the final 20-30 minutes
how is the dark adaptation curve obtained
by bleaching a substantial proportion of photopigment (exposing to very bright light & then putting them in a dark room) & then measuring their threshold over time at regular intervals
how is the shape of the dark adaptation curve affected
by how much photopigment is bleached to begin with i.e. how bright the adapting light is
what is a consequence to dark adaptation when the photopigment is more bleached to begin with i.e. very bright light
the longer it takes for them to dark adapt
what limits the threshold i.e. sensitivity of the retina during dark adaptation
rhodopsin breaks down into other photo products when it absorbs light and is bleached, so its the bleached properties of the retina that limits the sensitivity of the retina during dark adaptation
which photo products reduces the sensitivity of the retina & reduces the threshold during dark adaptation
opsin, retinal & metarhodopsin 2, becomes a bleached pigment and is colourless so can’t absorb light until it turns back into rhodopsin i.e. the retina then recovers its sensitivity
what is the reason for the dark adaptation curve
how long it takes for opsin & retinal to recombine
what does the Dowling-Rushton relationship state about the correlation between threshold during dark adaptation and concentration of bleached photoproducts remaining
Log sensitivity during dark adaptation is linearly related to concentration BLEACHED photopigment
what does Arden & Weale (1954) state about dark adaptation
Post-receptoral factors e.g. changes in receptive field size, also involved in dark adaptation
e.g. at the level of the outer plexiform layer to the inner plexiform layer can also influence dark adaptation, so mainly happens at level of photopigments, but a certain level of adaptation happens in the inner layers of the retina as well
which layers of the retina does regeneration of bleached visual pigment occur in
RPE & photoreceptor outer segments
what does regeneration of bleached visual pigment during dark adaptation require and from where in the retina
metabolites from choroidal circulation via bruch’s membrane
list 5 possible causes for delayed dark adaptation/poor night vision in relation to inability of regeneration of bleached visual pigment
- choroidal circulation abnormality
- Bruch’s membrane abnormality (is clogged up so nutrients can’t go from choroid to retina)
- RPE abnormality
- photoreceptor abnormality
- post-receptoral abnormality (inner retina)
which structures of the retina do you need for opsin & retinal to go back to rhodopsin
RPE & photoreceptors
in which structure does all trans retinal convert back to11-cis retinal
RPE
which 3 factors affect the rate of dark adaptation in people
- age
- retinal diseases e.g. AMD, retinitis pigmentosa, CSNB
- systemic conditions e.g. vitamin A deficiency
which 3 ways can you investigate and diagnose different causes of poor night vision
- psychophysical techniques
- fundus and anterior eye examination
- electrophysiological
which two clinical methods are there to measure dark adaptation
- goldmann adaptometer
- photostress test
explain how the goldmann adaptometer measures dark adaptation
- adapting light in ganzfield bowl
- all lights extinguished
- threshold measured at intervals post bleach
- takes more than 30mins for full dark adaptation curve
- every time px sees a spot of light, you use a pin to make a small hole in the paper
explain how the photostress test measures dark adaptation
- measure best va
- bleach with ophthalmoscope max aperture & max luminance on macula
- time taken to return to within 1 line of best va
- > 60 sec = pathological
- can be completed readily in practice by Optom
which clinical method to measure dark adaptation is quicker
photostress test
list two non-retinal causes of night blindness
- age related problems
- night myopia
what 2 age non retinal related problems can cause night blindess
- miosis
- lens opacities
both cause reduced retinal illuminance
how does night myopia cause night blindness
- eye focusses at about 1m in the dark (not at infinity)
- usually only when no visual stimulus, but possible when stimulus is degraded e.g. driving at night, as eyes may settle in a myopic state of about 0.50D
- may benefit from -0.50D over correction to balance
list four retinal causes of night blindness
- age
- ARMD
- chronic open angle glaucoma
- vitamin A deficiency
how does age relate to retinal problems in night blindness
reduced rate of photopigment regeneration causes slowed dark adaptation
which 3 ways does ARMD cause retinal problems in night blindness
- pathological worsening of normal ageing changes in retina
- rod cell death raises absolute threshold
- thickening of Bruch’s membrane, RPE and photoreceptor damage and reduced choroidal circulation slow pigment regeneration i.e. increase of lipofuscin & loss of photoreceptors
up to how much longer can an AMD px take to dark adapt than a healthy individual
30-50 min
what signs on the fundus show AMD
- mottled effect around the macula
- pigmentary changes around the macula
- build up of drusen
how does chronic open angle glaucoma relate to retinal problems in night blindness
ganglion cell loss coincides with area of max rod density (15 deg) - reduced scotopic sensitivity
how does vitamin A deficiency relate to retinal problems in night blindness
- Vit A important part of rhodopsin
- deficiency reduces rhodopsin concentration and slows photopigment regeneration once bleached
- results in raised absolute threshold and prolonged dark adaptation
which are the two inherited causes of night blindness
- retinitis pigmentosa
- congenital stationary night blindness (CSNB) - types 1, 2 and 3
which inherited condition causes gradual degeneration of night blindness
retinitis pigmentosa
which inherited condition causes stationary i.e doesn’t change over time degeneration of night blindness
congenital stationary night blindness
which method is used to differentially diagnose between retinitis pigmentosa and CSNB in relation to night blindness
dark adaptometry
what is the clinical presentation of congenital stationary night blindness type 1
normal fundus appearance
what is the clinical presentation of congenital stationary night blindness type 2
abnormal fundus appearance but can sometimes look normal
which two types of abnormal fundus appearances are there in congenital stationary night blindness type 2
- Oguchi’s disease
- fundus albipunctatus
what is the clinical presentations of congenital stationary night blindness type 3
- myopia
- nystagmus
- low vision
which type of genetics is linked to congenital stationary night blindness type 1
autosomal dominant, autosomal recessive & x-linked recessive
which type of genetics is linked to congenital stationary night blindness type 2
autosomal recessive
which type of genetics is linked to congenital stationary night blindness type 3
x-linked recessive
which type of congenital stationary night blindness is most common
type 1
aswell as normal fundus in congenital stationary night blindness type 1, whittles appears to be normal
visual acuity
what is affected/absent in congenital stationary night blindness type 1
rod branch of dark adaptation function is absent, cones less affected
what is affected in congenital stationary night blindness type 2
- rod and cone photopigment kinetics
- grossly extended adaptation times
- eventually normal thresholds - px has really slow dark adaptation, but if left long enough reaches normal thresholds
what appearance does the retina have with a px with congenital stationary night blindness type 2 fundus albipunctatus
dull white spots on retina
compared to a px who is dark adapted by half an hour, how long does it take for a px with congenital stationary night blindness type 2 fundus albipuncatatus to reach final threshold
3 hours
what appearance of fundus does a congenital stationary night blindness type 2 px with Oguchi’s disease have
grey/yellow metallic (but appearance normal after prolonged dark adaptation
what is normal with congenital stationary night blindness type 2 Oguchi’s disease
- regeneration of pigment
- cone adaptation
what is the abnormality with congenital stationary night blindness type 2 Oguchi’s disease
- post-receptoral abnormality
- rod adaptation grossly delayed 2-24 hours, then eventually normal thresholds
what is normal with congenital stationary night blindness type 3
cone dark adaptation
what is abnormal with congenital stationary night blindness type 3
rod phase of recovery absent
what does retinitis pigmentosa progressively cause in the retina
photorecessive degeneration, but progressive shortening of rod and cone outer segments due to abnormal disc shedding
what is the first symptom of retinitis pigmentosa
night blindness aged 20
what is later affected in retinitis pigmentosa
central vision
what fundus appearance does retinitis pigmentosa
- pigmented clumping in mid periphery
- attenuated blood vessels
which photoreceptor thresholds are grossly elevated in retinitis pigmentosa
both rod and cone & rod branch is missing = night driving really bad
